| Virtual Machine Generation ID Device |
| ==================================== |
| |
| .. |
| Copyright (C) 2016 Red Hat, Inc. |
| Copyright (C) 2017 Skyport Systems, Inc. |
| |
| This work is licensed under the terms of the GNU GPL, version 2 or later. |
| See the COPYING file in the top-level directory. |
| |
| The VM generation ID (``vmgenid``) device is an emulated device which |
| exposes a 128-bit, cryptographically random, integer value identifier, |
| referred to as a Globally Unique Identifier, or GUID. |
| |
| This allows management applications (e.g. libvirt) to notify the guest |
| operating system when the virtual machine is executed with a different |
| configuration (e.g. snapshot execution or creation from a template). The |
| guest operating system notices the change, and is then able to react as |
| appropriate by marking its copies of distributed databases as dirty, |
| re-initializing its random number generator etc. |
| |
| |
| Requirements |
| ------------ |
| |
| These requirements are extracted from the "How to implement virtual machine |
| generation ID support in a virtualization platform" section of |
| `the Microsoft Virtual Machine Generation ID specification |
| <http://go.microsoft.com/fwlink/?LinkId=260709>`_ dated August 1, 2012. |
| |
| - **R1a** The generation ID shall live in an 8-byte aligned buffer. |
| |
| - **R1b** The buffer holding the generation ID shall be in guest RAM, |
| ROM, or device MMIO range. |
| |
| - **R1c** The buffer holding the generation ID shall be kept separate from |
| areas used by the operating system. |
| |
| - **R1d** The buffer shall not be covered by an AddressRangeMemory or |
| AddressRangeACPI entry in the E820 or UEFI memory map. |
| |
| - **R1e** The generation ID shall not live in a page frame that could be |
| mapped with caching disabled. (In other words, regardless of whether the |
| generation ID lives in RAM, ROM or MMIO, it shall only be mapped as |
| cacheable.) |
| |
| - **R2** to **R5** [These AML requirements are isolated well enough in the |
| Microsoft specification for us to simply refer to them here.] |
| |
| - **R6** The hypervisor shall expose a _HID (hardware identifier) object |
| in the VMGenId device's scope that is unique to the hypervisor vendor. |
| |
| |
| QEMU Implementation |
| ------------------- |
| |
| The above-mentioned specification does not dictate which ACPI descriptor table |
| will contain the VM Generation ID device. Other implementations (Hyper-V and |
| Xen) put it in the main descriptor table (Differentiated System Description |
| Table or DSDT). For ease of debugging and implementation, we have decided to |
| put it in its own Secondary System Description Table, or SSDT. |
| |
| The following is a dump of the contents from a running system:: |
| |
| # iasl -p ./SSDT -d /sys/firmware/acpi/tables/SSDT |
| |
| Intel ACPI Component Architecture |
| ASL+ Optimizing Compiler version 20150717-64 |
| Copyright (c) 2000 - 2015 Intel Corporation |
| |
| Reading ACPI table from file /sys/firmware/acpi/tables/SSDT - Length |
| 00000198 (0x0000C6) |
| ACPI: SSDT 0x0000000000000000 0000C6 (v01 BOCHS VMGENID 00000001 BXPC 00000001) |
| Acpi table [SSDT] successfully installed and loaded |
| Pass 1 parse of [SSDT] |
| Pass 2 parse of [SSDT] |
| Parsing Deferred Opcodes (Methods/Buffers/Packages/Regions) |
| |
| Parsing completed |
| Disassembly completed |
| ASL Output: ./SSDT.dsl - 1631 bytes |
| # cat SSDT.dsl |
| /* |
| * Intel ACPI Component Architecture |
| * AML/ASL+ Disassembler version 20150717-64 |
| * Copyright (c) 2000 - 2015 Intel Corporation |
| * |
| * Disassembling to symbolic ASL+ operators |
| * |
| * Disassembly of /sys/firmware/acpi/tables/SSDT, Sun Feb 5 00:19:37 2017 |
| * |
| * Original Table Header: |
| * Signature "SSDT" |
| * Length 0x000000CA (202) |
| * Revision 0x01 |
| * Checksum 0x4B |
| * OEM ID "BOCHS " |
| * OEM Table ID "VMGENID" |
| * OEM Revision 0x00000001 (1) |
| * Compiler ID "BXPC" |
| * Compiler Version 0x00000001 (1) |
| */ |
| DefinitionBlock ("/sys/firmware/acpi/tables/SSDT.aml", "SSDT", 1, "BOCHS ", "VMGENID", 0x00000001) |
| { |
| Name (VGIA, 0x07FFF000) |
| Scope (\_SB) |
| { |
| Device (VGEN) |
| { |
| Name (_HID, "QEMUVGID") // _HID: Hardware ID |
| Name (_CID, "VM_Gen_Counter") // _CID: Compatible ID |
| Name (_DDN, "VM_Gen_Counter") // _DDN: DOS Device Name |
| Method (_STA, 0, NotSerialized) // _STA: Status |
| { |
| Local0 = 0x0F |
| If ((VGIA == Zero)) |
| { |
| Local0 = Zero |
| } |
| |
| Return (Local0) |
| } |
| |
| Method (ADDR, 0, NotSerialized) |
| { |
| Local0 = Package (0x02) {} |
| Index (Local0, Zero) = (VGIA + 0x28) |
| Index (Local0, One) = Zero |
| Return (Local0) |
| } |
| } |
| } |
| |
| Method (\_GPE._E05, 0, NotSerialized) // _Exx: Edge-Triggered GPE |
| { |
| Notify (\_SB.VGEN, 0x80) // Status Change |
| } |
| } |
| |
| |
| Design Details: |
| --------------- |
| |
| Requirements R1a through R1e dictate that the memory holding the |
| VM Generation ID must be allocated and owned by the guest firmware, |
| in this case BIOS or UEFI. However, to be useful, QEMU must be able to |
| change the contents of the memory at runtime, specifically when starting a |
| backed-up or snapshotted image. In order to do this, QEMU must know the |
| address that has been allocated. |
| |
| The mechanism chosen for this memory sharing is writable fw_cfg blobs. |
| These are data object that are visible to both QEMU and guests, and are |
| addressable as sequential files. |
| |
| More information about fw_cfg can be found in :doc:`fw_cfg`. |
| |
| Two fw_cfg blobs are used in this case: |
| |
| ``/etc/vmgenid_guid`` |
| |
| - contains the actual VM Generation ID GUID |
| - read-only to the guest |
| |
| ``/etc/vmgenid_addr`` |
| |
| - contains the address of the downloaded vmgenid blob |
| - writable by the guest |
| |
| |
| QEMU sends the following commands to the guest at startup: |
| |
| 1. Allocate memory for vmgenid_guid fw_cfg blob. |
| 2. Write the address of vmgenid_guid into the SSDT (VGIA ACPI variable as |
| shown above in the iasl dump). Note that this change is not propagated |
| back to QEMU. |
| 3. Write the address of vmgenid_guid back to QEMU's copy of vmgenid_addr |
| via the fw_cfg DMA interface. |
| |
| After step 3, QEMU is able to update the contents of vmgenid_guid at will. |
| |
| Since BIOS or UEFI does not necessarily run when we wish to change the GUID, |
| the value of VGIA is persisted via the VMState mechanism. |
| |
| As spelled out in the specification, any change to the GUID executes an |
| ACPI notification. The exact handler to use is not specified, so the vmgenid |
| device uses the first unused one: ``\_GPE._E05``. |
| |
| |
| Endian-ness Considerations: |
| --------------------------- |
| |
| Although not specified in Microsoft's document, it is assumed that the |
| device is expected to use little-endian format. |
| |
| All GUID passed in via command line or monitor are treated as big-endian. |
| GUID values displayed via monitor are shown in big-endian format. |
| |
| |
| GUID Storage Format: |
| -------------------- |
| |
| In order to implement an OVMF "SDT Header Probe Suppressor", the contents of |
| the vmgenid_guid fw_cfg blob are not simply a 128-bit GUID. There is also |
| significant padding in order to align and fill a memory page, as shown in the |
| following diagram:: |
| |
| +----------------------------------+ |
| | SSDT with OEM Table ID = VMGENID | |
| +----------------------------------+ |
| | ... | TOP OF PAGE |
| | VGIA dword object ---------------|-----> +---------------------------+ |
| | ... | | fw-allocated array for | |
| | _STA method referring to VGIA | | "etc/vmgenid_guid" | |
| | ... | +---------------------------+ |
| | ADDR method referring to VGIA | | 0: OVMF SDT Header probe | |
| | ... | | suppressor | |
| +----------------------------------+ | 36: padding for 8-byte | |
| | alignment | |
| | 40: GUID | |
| | 56: padding to page size | |
| +---------------------------+ |
| END OF PAGE |
| |
| |
| Device Usage: |
| ------------- |
| |
| The device has one property, which may be only be set using the command line: |
| |
| ``guid`` |
| sets the value of the GUID. A special value ``auto`` instructs |
| QEMU to generate a new random GUID. |
| |
| For example:: |
| |
| QEMU -device vmgenid,guid="324e6eaf-d1d1-4bf6-bf41-b9bb6c91fb87" |
| QEMU -device vmgenid,guid=auto |
| |
| The property may be queried via QMP/HMP:: |
| |
| (QEMU) query-vm-generation-id |
| {"return": {"guid": "324e6eaf-d1d1-4bf6-bf41-b9bb6c91fb87"}} |
| |
| Setting of this parameter is intentionally left out from the QMP/HMP |
| interfaces. There are no known use cases for changing the GUID once QEMU is |
| running, and adding this capability would greatly increase the complexity. |
